Process for treating metal



Dec. 26, 1939. w. A. DARRAH PROCESS FOR TREATING METAL Filed July 15, 1938 4 Sheets-Sheet 1 Dec. 26, 1939.

w. A. DARRAH PROCESS FOR TREATING METAL Filed July 15, 1938 4 Sheets-Sheet 2 fizz/222.507"

De 26,1939. DARRA 2,184,972

PROCESS FOR TREATING METAL Filed July 15, 1938 4' Sheets-Sheet 3 Dec. 26, 1939. D R 2,184,972

PROCESS FOR TREATING METAL Filed July 15, 1938 4 Sheets-Sheet 4 Patented Dec. 26, 1939 UNITED STATES PATENT OFFICE PROCESS FOR TREATING METAL William A. Dari-ah, Chicago, Ill.

Application July 15, 1938, Serial No. 219,332

Claim.

This invention relates to methods of treating metals and particularly the ferrous metals in-' cluding steels of various types. One important application of this invention is in connection with 5 the so-called carburizing of steel or various materials which consists in forming a case of highly carburized steel on the outer surface of an article.

This invention may be utilized to form other compounds or gases on a metallic base. as for example a nitride case and a mixture of carbide and nitride cases. Other compounds such as volatile metals as for example zinc may be added to the surface of the metal being treated some phases of my invention.

In order to simplify the disclosure I will confine the description of process and apparatus largely to the specific operation of carburizing steel articles, although it should be understood that my invention is by no means confined to this field only.

One of the objects of my invention is to form a strong hard resistant surface on a steel article. Another object of my invention is to form a surface on steel articles economically and rapidly.

Another object of my invention is to utilize naturalgas, city gas or the so-called liquid fuel gases such as butane and propane for carburizing operations without the production of excessive soot or coke on the metal being treated.

Another object of my invention is to provide means whereby various relatively "rich carbon bearing gases may be utilized for carburizing.

Another object of m I invention is to provide simple, automatic and economical means for completely carburizing and heat treating steel articles the operations including carburizing, reconditioning grain size, quenching, drawing, washing, etc.," so that the steel article when it leaves is completely treated.

I Other objects of my invention will be apparent from the drawings, description and claims appended hereto.

Referring to the drawings Figure 1 shows a typical plan view of one form of my invention.

Figure 2 shows a side elevation of the carburizing zone of this furnace partly broken away to indicate the interior construction.

Figure 3 shows a typical cross section through the furnace portion of my invention taken along line III- 111 of Figure 2.

Figure 4 shows a side elevation partially-broken away indicating quenching devices, discharge door and other details of my invention.

Figure 5 shows a side elevation partially broken utilizing fixture A recirculating draw oven 9 is shown located at the end of the I rinsing zone 8. 1

Similar numbers refer to similar parts on the various drawings and figures.

The loading platform 3 consists preferably of a structural steel frame work although obviously any suitable material may be employed.

A series of fixtures or trays III are loaded while resting on platform 3 and then pushed either manually or by the means of operating cylinder l l into vestibule 2 in the position shown in dotted lines and indicated by numeral l2.

At the proper time of the cycle, either controlled by the operator or automatically by the electric timer, operating pushing cylinder l3 away indicating a general arrangement of a draw pushes tray l2 into carburizing chamber I through operating door M. of course loading vestibule 2 is closed by manually operating door l5 to prevent leakage of air in or gases out during the cycle. This vestibule is also. supplied with purging gases, such as a mixture of products which are provided with a burner I8, burner block I9 and. exhaust fiue 20 which leads to a duct 2| located beneath the furnace or at some other convenient point. Radiant member I! naturally expands and contracts with changes of temperature and, therefore, is provided with a flexible seal or joint 22 which is filled with sand or other comminuted material. This joint is so constructed that a' complete metal contact is provided at all points except in the torus cups filled with sand. This construction provides for flexiof a strongly reducing atmosphere as this is ordinarily used to obtain the carburizing results desired. I have found that a porous refractory is of decided advantage for this service providing the outer face is well plastered with a refractory cement. All refractory material within the heating zone of the furnace should preferably be extremely low in iron oxide and similar compounds which may be readily reduced by temperatures ranging from 1600 to 1800 F. in the presence of hydrogen, carbon monoxide or other strongly reducing gases such as methane, and ethane, I have designated this layer of refractory material by the numeral 26.

A series of cross piers 21 are provided at intervals transversely across the furnace and between the radiating members. These should be made of strong materials and are used to support rails 28, 29 and 30, etc., on which the trays or other fixtures carrying the materials to be treated slide. The whole furnace is supported on a foundation indicated by 3|.

At the discharge end of furnace chamber I, a pusher'32 is located, arranged when actuated, to push the tray out of carburizing furnace I into cooling zone 4. In this zone the loaded trays are allowed to cool to temperatures ranging from 700 to 1200 or even over wider limits depending on the critical limits of the steel being treated. This chamber is tightly housed and consists substan-,

tially of a tight box or container hermetically sealed at one end to the carburizing furnace l,

, and at the other end to the reheat furnace 5.

Rails 33 are provided in this zone similar to those provided on loading table 3, so that as the trays are pushed out of chamber I they will slide on rails 33 until opposite pusher 34 and in the position shown in dotted lines by numeral 35. As the trays reach this point cylinder 34 is actuated pushing the trays out of cooling zone 4 through doorway 36 into reheating furnace 5, where the temperature is raised preferably to around l400 or higher to pass the critical temperature of the steel. When the tray reaches the end of reheat furnace 5 and assumes the position shown by the dotted tray 31, cylinder 38 pushes the tray into quenching chamber 6 as described. Quenching I chamber 6 is provided with an elevator member usually hot water with cleaning compounds (115- I 39 raised and lowered by cylinder 40. n the bottom portion of chamber 6 quench liquid such as oil or, if desired, water is maintained. By means of interlocks in the well known manner cylinder 40 lowers elevator 39 into the quench liquid 4| thus quickly cooling the metal and producing the desired metallurgical characteristics.

After a proper hesitation beneath the liquid to permit cooling of the steel to the desired temperature cylinder 40 lifts the elevator and tray to the operating level at which time pusher 42 is actuated and the tray is pushed into the washing zone 1. This washing zone consists of a series of sprays supplied with washing liquid which is piers solved therein. A pump 43 rotated by a motor 44 delivers the liquid to the spray nozzles at high pressures and large volumes as required. This type of equipment is well known.

The trays are pushed successively the length of one tray passing gradually through the washing zone and the rinsing zone, allowing ample time to remove oil, grease, carbon, dirt, coke and other foreign materials.

The rinsing zone is provided with a motor 45 and pump 46 which serves to supply clean water for removing all traces of washing compound. If this is not done the compound is likely to dry on the steel parts causing an unsightly appearance. I prefer to use washing liquids strongly heated preferably close to 200 F. This, however, is an operating detail which may be varied to suit working conditions.

After leaving the rinsing zone the trays are pushed successively and intermittently through a draw oven as indicated in Figure 5.

In this oven the material is reheated gradually and uniformly to temperatures ranging from 300 to- 700 or 800 depending again on the nature of the steel and the results to be obtained. should be understood that wherever temperatures are specified they are given with a view of obtaining great toughness and hardness of the outer layer of carburized steel and toughness and strength in the inner layer of steel. Other results may be obtained by this invention and are not to be excluded. Other results may require difierent operating temperatures and times.

The draw oven 9 shown in Figure 5, is supplied with circulating hot gases from duct 50 indicated in Figure 1. Duct 50 is arranged to receive the products of combustion from both the carburizing furnace l and the reheat furnace 5. These products of combustion pass from the radiant combustion chambers I! through the flexible joint 22 into down-comer 20 and finally connect to longitudinal flues 2| which run the entire length of the furnaces. Duct 5i connects with the longitudinal duct beneath the carburizing furnace and duct 52 connects with the duct beneath the reheat furnace. Obviously these ducts may be put above the fioor'or beside the furnaces if it' is found more convenient to do so. Duct 5| is provided with a draft control damper 53 and duct 52 has a similar draft control damper 54. The two ducts connect to riser 55 which in turn leads the combined products of circulating combustion into duct 50 and then into the intake of fan 56 which delivers the hot gases into draw oven 9 at a point 51 located beneath the rails 58 on which the trays 59 are carried. Since the trays are of opened or' latticed construction the hot gases passing into the oven in space 51 travel upward, around and through the interstices of the trays and leave. by means of spaces 60 between plate 6! which form the upper portion of the draw oven. The circulating gases thus enter plenum chamber 62 from which they pass toduct 63 which leads them back again to the recirculating fan. A control damper 64 in duct 50 may be operated by a thermostatically controlled motor in any conventional manner and thus draws more or less products of combustion into the circulating fan to maintain the temperature of the circulating gases constant.

A thermostatic device 65 such as a sensitive couple placed in the circulating air stream in passage 51 serves to open or close damper 64 as required.

It will be evident that the hot circulating gases which utilize waste heat from the furnaces serve to give a closely controlled draw to the steel parts which have been carburized, cooled, reheated and quenched.

This oven also serves as a drying oven to remove the water from the washing, and rinsing process and may be so used exclusively, if it is not desired to draw and temper in the steel pot. In mostcases the drawing operation serves to toughen and soften the inner core of steel thus increasing the strength and durability of the product. Since the carbon content of the carburized shell on the outer surface of the steel is higher than the carbon content of the core, temperatures which will draw the core satisfactorily will not soften the shell appreciably.

After the trays have been pushed successively through the draw oven occupying in turn each of the positions indicated they finally come to the positions shown as 66 in Figure 5, after which they are pulled or pushed into vestibule 61. Vestibule 61 serves as a seal or hood to prevent excessive leakage in or out of draw oven 9. Succeeding position 68 provides a place for the trays to 0001 before unloading and reloading.

It will be apparent that the trays have thus made the complete cycle of the apparatus having been submitted consecutively to each of the operations. Since the timing is controlled by an automatic timer or electric clock all times in carburizing, cooling, reheating and drawing are accurately controlled. Furthermore, since temperatures and atmospheres are closely controlled the entire process gives a uniform product having predetermined characteristics.

Referring to Figure 6, tray 10 may be taken as a typical unit used in my process but I do not Wish to be restricted to this particular design as any commercial trays properly suited for this process may be employed.

The tray 10 consists of a frame H which is sumciently rugged to permit satisfactory pushing of the entire load. The frame ll surrounds a lattice indicated by 12 formed from a number of transverse and longitudinal bars. Preferably the entire tray is cast in one single integral piece, but if desired it may be formed in several sections dovetailed or otherwise fastened together.

At the junction of the bars forming the lattice or at other points as required, a series of hub or supporting members indicated by 13 are provided. Any one of these members may be formed on the tray to suit the loading conditions required. A series of fixtures 14 may be provided to slip into the hollow; portion of hubs similar to 13 and these fixtures 14 support gears or other parts indicated by 15. It is understood that the trays will be loaded as desired and a typical installation may represent a load of 800 or 1000 pounds of steel to each tray which may weigh, by way of example, 200 to 300 pounds. Naturally these ratiosand arrangements will vary widely with the nature, size and type of articles being treated and the kind of treatment given.

It will be apparent that in the process here described products of combustion are excluded from the chamber in which the steel articles are heated so that it is possible to absolutely control the nature of the atmosphere in contact with the steel being treated. This is an essential feature of this invention in that proper control of the chemical and physical condition of the atmosphere is essential for obtaining the results desired.

I am aware that carburizing has been performed by heating steel articles in a closed chamber in contact with carburizing gases. This process, however, has not met all requirements for numerous reasons among which are slowness of forming a desired case, difliculty in controlling the type and depth of case and the presence of large amounts of soot or coke on the surface of the steel. t

Various expedients have been proposed to overcome some of these difiiculties but from my practical experience in this field I find that these expedients are not satisfactory.

In my invention I introduce carburizing gases into my furnace both above and below the trays carrying the work, as for example, by means of upper inlets 1B and lower inlets I1. I provide a number of these inlets along the length of travel of the work preferably placing them every three or four feet apart for purposes of control. These inlets may be connected to a suitable source of carburizing gas. gas, propane or city gas direct without dilution or addition when it is desired to produce a carbon case on the surface of the work. In the past such a procedure would coat the surface of the steel heavily with soot or coke. This coating has several obvious objections. In the first place, it is difficult to remove andrequires costly and expensive handling or polishing to entirely remove it as it is strongly adherent. If it is not fully removed I prefer to use either natural it naturally causes serious wear in the devices utilizing the gears or other equipment treated.

A further objection to the soot is the fact that it apparently delays and retards the penetration of carbon into the steel.

In my process I have found that while I provide a number of inlets for the natural gas both above and below the work, it is preferable to introduce the gas principally at the discharge end of the carburizing furnace at a point where the articles are about to leave the treatment. The carburizing gas will flow backward toward the loading end and promptly fill the entire chamber with atmosphere.

' If gas is introduced the full length of the chamber sooting is likely to occur in the early stages with consequent slowing up of the carburizing reaction or a reduction of the percentage of carbon in the case.

Carbon of course first collects in the outer layer of steel and gradually penetrates to the interior of the steel. The concentration of carbon is naturally higher in the outer layers initially and lower in the inner layers. Most articles require for maximum hardness and durability a layer of hyper eutectoid in the outer layer, just below this a strata of eutectoid and below this a final layer of hypo-eutectoid. If soot deposits occur they may retard the addition of carbon to the outer layer but of course they do not retard the diffusion of carbon from the outer layer into the interior of the steel. Soot deposits, therefore, may be objectionable by causing a lower percentage of carbon in the case distributed over the entire band.

By my process the carburizing gas, as for example natural gas, may be partly above the material being treated and partly below. The gas as added eventually comes in contact with the highly heated radiant chambers made of alloy or other heat resisting material. These chambers may range from 1800 to 2000 or over and serve to crack the methane and other hydrocarbons in the gas. The degree of cracking depends on the time of contact, the temperature of which the gas is heated and the velocityof circulation of the atmosphere within the carburizing chamber. Naturally, having an intensely hot zone in the lowerportion of the furnace violently circulating atmosphere streams are produced, and the atmosphere within the carburizing chamber is kept well mixed and in rapid motion.

The breaking down of methane and other hydrocarbons naturally deposits some free carbon around the combustion chamber and the adjacent refractory walls and produces large amounts of hydrogen. The hydrogen serves as a dilutent for the incoming new quantities of methane in the natural gas and therefore protects it against rapid decomposition by the well known law of mass action.

As a result of this condition it is only necessary to provide a proper ratio of hydrogen and inert gases to the amount of-methane nad hydrocarbon gases so that the deposit of'carbon on the steel is reduced to a rate approximately equal to that at which it will be absorbed by the steel.

Further the hot cracked gases appear to be in a nascent or active condition and a greatly accelerated rate of carburization of the steel results.

It should be understood that while I have described my process as applied to steel it may also be used with tungsten, chromium and other metals.

,It should be understood that the statements made in this specification relating to reasons for obtaining the specially advantageous results which are secured, represent my theory as to operating principles and are not necessarily actual facts:

It is apparent that there is a rapid and violent circulation throughout the zone of the furnace occupied by the steel being carburized. It is also evident that the temperature of the gases doing the carburizing is higher than the temperature of the steel since the gases pass over the intensely hot radiating chambers at the bottom of the furnace and travel upward around the work being carburized. These two points I believe are of great importance in reducing the time required for carburizing and increasing the depth of case and the amount of carbon absorbed.

These factors also make it possible to carburize with less additional atmosphere than is required in the case of present day muiile type furnaces in which the carburizing chamber is of necessity maintained at a relatively uniform temperature at the top and bottom of the chamber. The present carburizing chambers or mufiles being made of alloy and, therefore, very costly are kept as small as possible. It is necessary to keep the present carburizing furnaces small in order to reduce the expansion and warping which is an extremely serious item in the case of a large muiile. In the case of the furnace which I have invented there is no large mufile but merely a brick wall which is ordinarily formed of insulating refractory plastered with a refractory cement to seal the pores against the entrance of the carburizing gas. As a result of my construction many of the limitations of the present furnaces are removed and I can build a furnace of my design in any desired size in any dimension, using commercial materials now available. This is not practical with present day carburizing mufiles, owing to the limitations of making and maintaining large alloy boxes or containers which of course .must be kept air tight.

In the case of equipment of my design, making a longer furnace merely means building the steel container of greater length and lining it with flrebrick as shown. More of the small individual radiating units are then added as required. Obviously the same principle applies to increasing the width of my furnace.

By introducing the new carburizing atmosphere which may be methane, propane, butane or various other hydrocarbon gases or a mixture of these, or hydrocarbon containing CO, nitrogen,

etc., at the upper portion-of my heating zone, I

obtain one set of conditions and I obtain a different set of conditions by introducing the carburizing gases in the lower portion of the heating zone. By controlling the amountintroducedabove and below the work to be carburized I have a type of control of the nature of the case formed on the steel, which is not possible in present day carburizing units.

I prefer to exhaust a small amount of the gases at each end of the carburizing furnace partly to provide space for introducing new gas and partly to provide a visual indication of the nature of the gas within the furnace. By lighting the stream of gas which leaves the furnace at the ends, I am able to produce a flame which has distinctive characteristics depending on the composition of the gas. Thus an extremely rich gas will burn with a deep red or reddish yellow flame, producing considerable smoke, while a leaner gas will produce a lighter colored flame and a gas of hydrogen and CO willproduce a flame which is still more transparent. It has been the experience of operators using my equipment that as long as the nature of the flame is held constant the gas condition within the furnace is substantially constant and the nature of the case produced on the steel is substantially constant.

Another point which I have found of great advantage in the furnace which I have invented is the fact that the necessary temperatures for producting a given degree of case, other things being equal, are considerably less in my furnace than in the case of the older type of muffle.

I attribute this fact partly to the rapid circulation of the gases due to the thermal difference caused by an intensely hot radiating member in the bottom of the furnace and partly to the fact that the gases passing over the steel or other material being treated are hotter than the steel itself.

It will be understood that my invention includes a process of carburizing and also the equipment for accomplishing this result. My invention includes also a cycle and process as well as the associated equipment, in which the waste heat from the high temperature operation is utilized in the low' temperature operation.

My invention also includes full automatic equipment and various other features as set-forth in the specifications and the attached claims.

It will be. understood I do not wish to be restricted to any special set of temperature, time or gas conditions as these are ordinarily varied to suit the requirements of the material being treated. I have found, however, that my process and equipment permits substantially shortening the time, lowering the temperature and increasingthe depth and carbon content of the case. The relative degree to which these different advantages exist are subject to control depending on conditions to be obtained. 4

By adding ammonia to the atmosphere which I use for obtaining the carburized case I am able to add a substantialamount of ni rogen to the am able to produce a case which contains bothnitride and carbide and thus has many-of the characteristics ordinarily associated with the treatment of steel in a molten bath of sodium cyanide.

As indicated above, I may use as atmosphere straight natural gas without adding any form of diluting material. Under these conditions the cracking of the gas liberates hydrogen, a portion of the carbon being absorbed by the steel and a portion of the carbon being precipitated as coke or graphite, some on the walls of thefurnace and some on the heat radiating member or other parts. The presence of this accmnulation of carbon appears to ultimately retard the cracking of the methane in the natural'gas thus preserving more of the gas for use in forming the carburizing case on the steel. This is my theory as to one of the reasons why I am able to carburize a given quantity of steel to a given depth with much less carburizing gas than is ordinarily used in present day conventional furnaces.

It will be apparent, therefore, that the atmosphere within my furnace will ordinarily contain a considerable amount of free hydrogen which has proved to be of no objection in the case of any commercial application so far tested.

In the opening of the doors for loading and discharging ofmy furnace there is naturally a slight leakage of gas outward from the doors and of air into the furnace chamber. This has proved to cause no difficulty but accounts for the presence of a small amount of nitrogen and oxides of carbon within the furnace chamber. Of course vestibules are provided as indicated to reduce this condition to a minimum but'my experience has been that quite substantial amounts of air may enter the furnace chamber without causing difficulties.

Water vapor may be present in a small degree but it is desirable to exclude water vapor as far as possible when a high carbon content is desired on the surface of the steel.

I wish to emphasize that in the process and equipment which I have invented I maintain an intensely hot source of radiation in the lower portion of the carburizing chamber while the material being carburized or otherwise treated is located in another portion of the chamber, usually the 1 upper portion. As a result of this condition, the

atmosphere within the carburizing chamber comes in contact with intensely hot surfaces, acquires a portion of this-intense-heat and then travels upward by thermal circulation, coming in contact with the cooler steel or other articles being carburized. I As a result of this condition, the hydrocarbon gases are hotter than the steel which is being carburized, even when the steel is up to its 5, than on the steel.

maximum temperature. This condition causes the soot, which is precipitated on the steel in other competitive processes now in use, to deposit in those portions of the furnace which are hot rather This condition leaves the steel relatively clean and free from deposits of coke and soot, which are characteristic of other methods of carburizing now in use. The deposit of 'coke or soot on the surface of the steel is a erious disadvantage from many standpoints. It tretards'the rate of carburization because it forms a coating or shield and keeps the carburizing gases from having intimate contact with the steel. Naturally a heavy coating of soot or coke requires a cleaning and in the case of present commercial process this cleaning is quite difllcult and costly.

It will be noted that in present commercial processes an alloy mufile forms. the containing chamber in which the steel being treated is enclosed together with the carburizing gas. It is necessary. to heat the mufile throughout its cross section to a fairly uniform temperature to avoid excessive warping, cracking and deterioration.

This condition means that the rapid thermal cir- 7 culation is missing and the temperature of the muflle is substantially the temperature of the steel being treated. Therefore, present commercial processes lack the substantial difference in temperature between the steel being carburized and the gas containing the carburizing material. It appears that the desired results can only be obtained in the best measure by utilizing the thermal circulation which this process and type of construction provides. Forced circulation by fans does not give the results obtainable by this process, possibly because the difference in temperature between the circulating gases and the material being carburized is not as great as in the case of the thermal circulation which I utilize.

,It is notable that articles treated by my process come out clean and free from coke and contain novisible soot. While I am familiar with many of the present day carburizing processes I have found none which give this condition. It should be noted that this condition is obtained without the addition of foreign gases of any kind and no oxidizing agents or oxidizing gases are essential to my process although they may be present to a limited degree without interfering with the operation of my process.

A substantial portion of my invention, therefore, is the means of obtaining clean and uniformly carburized articles in a minimum time and with a lower temperature than is' customary today.

Owing to the rapid thermal circulation of the hot carburizing gases and the fact that they are at a higher temperature than the metal being carburized, carbon is added with greatuniformity Q which is not the case with mufile furnaces now in use.

Having now fully described my invention, what I claim as new and wish to secure byLetters' Patent in the United States, is as follows: 1

1. A process of carburizing metals in a heated chamber by treating with a hot gaseous atmosphere containing two different types of constituents, one constituent consisting of relatively inert gases and the second being active as a carburizing agent, said process including the step of introducing a hydrocarbon into said chamber and heating same'by passing it into proximity to a source of heat so as to condition said hydrocarbon before it comes into contact with the metal to be carburized and then transferring the conditioned hydrocarbon into contact with said metal by thermal circulation.

2. In the process of carburizing metalsby means of a gaseousatmosphere containing a relatively inert and arelatively active gaseous constituent, the step of preconditioning a portion of hydrocarbon gas by strongly heating and thereby conditioning it before bringing said hydrocarbon into contact with the metal to be carburized, mix- 1 ing said pretreated hydrocarbon with a'portion of said relatively inert gaseous constituent and bringing said mixture into contact with said metal while at a temperature within the carburizing range.

3. A process of carburizing metals by a gaseous atmosphere in a chamber with walls of nonmetallic refractory relatively free from readily reducible metallic compounds, said process comprising the use of a hot controlled mixture of a relatively inert gas and a relatively active carburizing gas said mixture being thermally recycled about the metal to be carburized, said active carburizing gas being formed by heating a portion of hydrocarbon gas to a temperature within the carburizing range before contacting said metal and said inert gas consisting largely of the hot spent products from said active gas after utilization.

4. A process of controlling the operation of carburizing metals in a heated chamber, while exposed to a hot gaseous atmosphere containing two constituents, one constituent consisting of gases relatively inert from a carburizing standpoint, and the other being active as a carburizing agent, said process including the step of continuously introducing a controlled amount of hydrocarbon into said chamber and conditioning same by passing it into proximity to a source of heat, before it comes into contact with the metal to be carburized, and then continuously transferring said hot conditioned hydrocarbon into contact with the metal to be carburized by thermal circulation, and controlling the temperature of said metal during treatment.

5. A process of carburizing metals by a hot gaseous atmosphere in a heated chamber, said process comprising the use of a controlled mixture of a relatively inert gas and a relatively active carburizing gas said mixture being thermally recycled about the metal to be carburized while said mixture is hotter than said metal, thereby contributing to the heating of said metal while carburizing, the said active carburizing gas being formed by heating a portion of the hydrocarbon gas before contacting said metal to be treated.

WILLIAM, A. DARRAI-I. 

